This invention relates to airborne navigation receivers and particularly to receivers for deriving aircraft azimuth or elevation data through reception of information from a scanned beam subject to asymmetry
The current Microwave Landing System ("MLS") specified by international standards operates by providing angular position information to an aircraft receiver by means of fan-shaped beams which are scanned "TO" and "FRO" across a spatial coverage sector by ground based transmitting equipment. The airborne receiver uses the time of each beam scan to measure the elapsed time between successive TO and FROM scans in each cycle. This time difference is directly proportional to the aircraft's angular position in azimuth or elevation and when processed with distance information provided by associated MLS distance measuring equipment ("DME") the aircraft position in space can be determined.
In an article entitled "Comparison Study of MLS Airborne Signal Processing Techniques", by Kelly and LaBerge, IEEE NAECON "78", May 1978, the characteristics of different types of MLS receivers were considered, including receivers using dwell gate and split gate signal processing techniques. In addition to discussing structure and operation of such receivers, the article addresses the effects of thermal noise and multipath signal reception and the errors caused thereby, but does not consider effects of beam asymmetry.
Beam asymmetry can result from various causes. It is inherent in the azimuth transmission at wide scan angles as a result of the electronic scanning of a horizontal line array antenna and the fact that, in the MLS coding, angle data is linearly related to the azimuth angle rather than to the sine of the azimuth angle. Applicant has determined on the basis of analysis and computed results that, for a three degree beamwidth azimuth beam, at a scan angle of 50 degrees a prior art split gate processor receiver could be subject to a 0.05 degree azimuth error. In elevation, asymmetry results basically from designing the transmission equipment for a minimum size elevation array antenna through reduction of bottom-side sidelobes and increase in power of top-side sidelobes. For antennas optimized in this way, errors in elevation could reach the maximum allowable pursuant to the International Civil Aviation Organization ("ICAO") beam shape specification for MLS, representing errors of 0.075 degrees for a split gate receiver. It should be noted that ICAO standards state "the airborne equipment shall not degrade the accuracy of any decoded angle guidance signal by greater than .+-.0.017 degrees."
With reference to FIG. 1A, it should be noted that the ICAO standards define the beam center as lying midway between the points on the beam envelope 3 dB below the beam peaks (the "-3 dB points") and provide tolerances for the separation of the -10 dB points from that center line, thereby inherently permitting beam asymmetry. There is no specification for the location of the beam peak. As shown in FIG. 1B, for a hypothetical asymmetrical beam the beam center line is still midway between the -3 dB points, by definition, but at the -10 dB level the point midway between the -10 dB envelope intersection points is displaced from the defined center line based on the -3 dB points. Thus, a receiver measuring beam center at the -3 dB power level will not be subject to an error resulting from beam asymmetry, but receivers relying on beam center determinations made at other power levels will be subject to errors caused by the lack of beam symmetry. In FIG. 1B, line A represents the defined beam center, line B is the actual beam center line of the asymmetrical beam illustrated, displacement C represents the error between actual and defined beam centers and D is the beam envelope.
In order to reduce receiver sensitivity to thermal noise or multipath reception effects or for other reasons it may be desirable to utilize receiver designs which operate on the basis of measurements involving power levels other than a defined standard power level such as -3 dB, however, it has not been possible to do so independently of errors caused by beam asymmetry. It should further be noted that ICAO has recognized that such asymmetry errors are inherent in the MLS beam as specified and has determined that beam specifications will not be revised to change the permissible levels of beam asymmetry.
It is therefore an object of this invention to provide receivers substantially immune to angle determination error resulting from beam asymmetry and MLS receivers capable of determining aircraft position with increased accuracy.